This application claims the priority of German Application No. 198 15 928.5, filed Apr. 9, 1998, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a semiconductor igniter, particularly for the gas generator of a vehicle occupant protection system and, more particularly, to a semiconductor igniter having a semiconductor layer which is arranged on a carrier with the insertion of a thermal insulation layer and is connected at an end side to electric contact areas such that during the current passage in the ignition path range it heats up in an ignition-triggering manner.
Semiconductor igniters of this type, which, in contrast to hot wire igniters, are becoming more prevalent mainly because of their significantly lower interference susceptibility, are known from European Patent document EP 0 762 073 A1 or from U.S. Pat. No. 5,309,841. The semiconductor igniters consist of a considerably p-doped or n-doped semiconductor layer which is arranged between end-side contact pieces on an electrically insulated or non-conducting carrier. During the current passage, while generating an ionized semiconductor plasma, the igniter abruptly heats up and evaporates and, as a result, triggers the ignition--mostly by way of a primary ignition charge. For reasons of a high ignition efficiency, it is required to insert a thermal insulation layer between the semiconductor layer and the carrier. However, as a result, the mechanical bond from the semiconductor layer to the carrier is impaired, and there is the danger that the semiconductor layer may detach under the effect of thermal or dynamic loads as they occur particularly in an application in a motor vehicle, and the semiconductor igniter therefore becomes inoperative.
It is an object of the invention to construct a semiconductor igniter of the above-mentioned type such that a high constructive stability is achieved in a manner which is simple with respect to the manufacturing of the igniter while maintaining a high ignition efficiency.
According to the invention, this object is achieved by a semiconductor igniter, particularly for a gas generator of a vehicle occupant protection system, having a semiconductor layer which is arranged on a carrier with the insertion of a thermal insulation layer and is connected at an end side to electric contact areas such that during the current passage in the ignition path range it heats up in an ignition-triggering manner. The thermal insulation layer is limited to the ignition path range of the semiconductor layer and, on its end sections kept free of the thermal insulation layer. The semiconductor layer is fixedly connected with the carrier.
According to the invention, as a result of the spatial limiting of the thermal insulation layer to the ignition path range of the semiconductor layer in conjunction with a linking of the bridge end sections directly to the support, which link is of the same material and is therefore correspondingly firm, a support of the semiconductor layer is ensured which is extremely stable with respect to the occurring loads. Further, the operational reliability of the semiconductor igniter is significantly improved without additional high-expenditure measures. Nevertheless, the thermal shielding of the ignition path range, which is required for a high ignition efficiency, is fully maintained.
In a particularly preferred further embodiment of the invention, the semiconductor layer is molded on the end sections in one piece to the carrier, whereby an even more secure bond is ensured between the semiconductor layer and the carrier.
For further increasing the stability with a simultaneously high thermal protection effect, it is recommended to produce the thermal insulation layer of a porous material which supports the semiconductor layer in the ignition path range, specifically in a manner which is simple with respect to manufacturing. Here, the carrier material itself is locally made porous, for example, electrochemically. In this case, the material, which is made porous, is preferably oxidized in order to further reduce the thermal conductivity of the insulation layer.
However, optionally, it is also possible to construct the semiconductor layer preferably as a bridge structure which is free-standing in the ignition path range; specifically expediently such that the insulation material, which was first made porous, is removed by etching so that a hollow space is formed as the thermal insulation layer which reaches under the ignition path range. The hollow space is filled with air and can be evacuated as desired. This still further reduces the thermal ignition energy losses.
In a particularly preferred manner, the semiconductor layer is surrounded in the ignition path range by an ignition intensifying medium which burns in an explosive manner when heated, whereby, after a relatively low temperature level has been reached, non-electrically generated heat is made available to the ignition process. According to the invention, the ignition intensifying medium is expediently applied to the semiconductor layer in the form of a coating which is thin with a view to obtaining a short ignition delay. However, when using a porous insulation layer, it is, optionally or additionally, also possible to impregnate the porous insulation material with a gaseous or metal-containing ignition intensifying medium for intensifying the ignition pulse.
According to a preferred embodiment of the invention, the semiconductor layer is divided into several mutually parallel bridge-type webs which are thermally insulated with respect to one another and with respect to the carrier. As a result, in the case of a comparatively large bridge width which is advantageous for creating large contact surfaces for the ignition charge situated above the semiconductor layer, by way of the spaces existing between the bridge-type webs, a thermal insulation layer can be constructed without any problem on the bridge underside.
In a particularly preferable manner, the semiconductor layer is constructed as a semiconductor element which is operated in the blocking direction, heats up in an ignition-triggering manner when the switching voltage is exceeded and has at least a p-n transition junction, thus approximately as a pair of diodes connected antiparallel. This further reduces the interference susceptibility of the semiconductor igniter and produces a pronounced short, sharp ignition pulse.
According to the present invention, the carrier and the semiconductor layer are preferably produced from differently doped silicon, for example, in the form of a silicon wafer.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.